ASSAY OF RADIOACTIVITY 



vapour, others are low-voltage halogen-quenched tubes; the relative merits 

 of the two classes have already been discussed. 



End-v^'indow tubes are normally used in lead castles fitted with shelves for 

 supporting at various heights beneath the window a tray carrying the radio- 

 active sample in solid form. The samples are usually mounted on thin metal 

 planchettes. With this arrangement the top shelf brings the sample close 

 enough to the window for up to 30 per cent of the radiation emitted to reach 

 the sensitive volume. The lower shelves are useful if the samples are giving 

 excessively high counting rates or if it is necessary to insert a filter between 

 the sample and the tube in order to cut out weak radiation in experiments 

 involving mixtures of different isotopes. This type of mounting can also be 

 adapted for use with automatic sample changers, in which a succession of 

 samples is brought under the tube by a mechanical device, and the counts are 

 recorded automatically. 



End-window tubes are also convenient for a variety of other purposes. 

 Thus they may be used to scan paper strips on which labelled compounds 

 have been separated by chromatography. They may be used for general 

 monitoring. They may be applied externally to living animals in order to 

 locate concentrations of radioisotopes within the animal (e.g. in experiments 

 on circulation time, or the uptake of isotopes by different organs), or they 

 may be used to measure changes in the amount of radioactivity in isolated 

 living tissues, mounted in a thin-bottomed chamber in a stream of inactive 

 fluid (an example of this type of application is described by Keynes^^). 



Liquid counters 



It is sometimes more convenient to count samples in liquid than in solid 

 form, and on such occasions a liquid counter may be employed. This 

 incorporates the kind of thin- walled Geiger tube illustrated in Figure 31.4, 

 immersed in a tube of such a diameter that the liquid occupies an annulus 

 about 2 mm wide surrounding the sensitive volume. In the version designed 

 by Veall^*, which is available commercially from several of the makers 

 listed in the previous section, the outer glass tube is fused with the Geiger 

 tube to form a fixed skirt, this arrangement yielding a liquid counter with 

 admirably reproducible geometry. These skirted tubes can be operated in 

 two ways. About 9 ml of liquid is enough to cover the thin-walled part of 

 the Geiger tube, and the addition of further active fluid above this level has 

 little effect on the counting rate; if, therefore, it is used with about 10 ml of 

 fluid, it measures the specific activity of the fluid. Alternatively, if a small 

 volume (say 3 ml) of active fluid is poured into the counter and then progres- 

 sively diluted by adding inactive fluid, it is found that the counting rate 

 reaches a fairly flat maximum at a total volume around 6 ml. For determin- 

 ing the total activity in a sample whose precise volume need not be known if 

 it is less than 6 ml, the tube can therefore be used by pipetting the sample 

 into it, making up to a mark at 6 ml with inactive fluid, and mixing. Small 

 variations in making up to the mark cause negligible errors. 



Owing to absorption of radiation in the fluid, liquid counters are unsuitable 

 for use with weak /5-emitters like ^*C and ^^S, but give reasonable counting 

 efficiencies for /)-radiation stronger than 0-5 MeV, or for y-radiation. They 

 give an effective solid angle for counting of nearly Itt, and for a very strong 



436 



